Many Shanghai residents take fresh water supplied to their homes for granted; the tap is turned on and the water begins to flow.

But few are aware that a group of dedicated armed police officers safeguard the Qingcaosha Reservoir, the city's largest drinking water source, 24 hours a day, seven days a week.

"We are always on alert," said Lu Xiang, head of the police substation in the Qingcaosha area, west of Changxing Island at the mouth of the Yangtze River.